Use of mixtures of monocarboxylic acids and polycyclic hydrocarbon compounds for increasing the cetane number of fuel oils

ABSTRACT

The use of mixtures of (A) aliphatic saturated or unsaturated monocarboxylic acids having 12 to 24 carbon atoms or the dimerization or trimerization products thereof, which may be present in the form of free carboxylic acids and/or in the form of ammonium salts, amides, esters and/or nitriles, and (B) polycyclic hydrocarbon compounds which are obtainable from distillation residues of natural oils, which have been extracted from tree resins, for increasing the cetane number of fuel oils which comprise at least one additive with detergent action and at least one cetane number improver, the mixtures of components (A) and (B) being used in a concentration of 10 to 500 ppm by weight, based on the total amount of the fuel oil.

The present invention relates to the use of mixtures of aliphatic saturated or unsaturated, relatively long-chain monocarboxylic acids or derivatives thereof and polycyclic hydrocarbon compounds for increasing the cetane number of fuel oils which comprise at least one additive with detergent action and at least one cetane number improver.

Fuel oils generally comprise cetane number improvers, which are also referred to as ignition accelerators or combustion improvers. For this purpose, typically organic nitrates are used, which have been known for some time as cetane number improvers in fuel oils or middle distillates such as diesel fuels, and have also been used therein.

Higher cetane numbers lead to more rapid engine starts, especially in cold weather, to lower engine noise, to more complete combustion, to less evolution of smoke and, under some circumstances, to lower injector carbonization.

Typical organic nitrates which are suitable as cetane number improvers in fuel oils, especially in diesel fuels, are nitrates of short- and medium-chain, linear and branched alkanols and nitrates of cycloalkanols, such as n-hexyl nitrate, 2-ethyl-hexyl nitrate, n-heptyl nitrate, n-octyl nitrate, isooctyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, n-dodecyl nitrate, cyclopentyl nitrate, cyclohexyl nitrate, methylcyclohexyl nitrate and isopropylcyclohexyl nitrate. Specific branched decyl nitrates of the formula R¹R²CH—CH₂—O—NO₂ in which R¹ denotes an n-propyl or isopropyl radical and R² a linear or branched alkyl radical having 5 carbon atoms are also recommended in WO 2008/092809 as combustion improvers or cetane number improvers. However, the most commercially important cetane number improver is 2-ethylhexyl nitrate.

However, the prior art cetane number improvers mentioned are still in need of improvement in terms of action. It was thus an object of the present invention to increase the cetane number of fuel oils, especially of diesel fuels and of mixtures of biofuel oils and middle distillates of fossil, vegetable or animal origin, by a suitable measure.

Accordingly the use has been found of mixtures of

-   (A) aliphatic saturated or unsaturated monocarboxylic acids having     12 to 24 carbon atoms or the dimerization or trimerization products     thereof, which may be present in the form of free carboxylic acids     and/or in the form of ammonium salts, amides, esters and/or     nitriles, and -   (B) polycyclic hydrocarbon compounds which are obtainable from     distillation residues of natural oils, which have been extracted     from tree resins, for increasing the cetane number of fuel oils     which comprise at least one additive with detergent action and at     least one cetane number improver, the mixtures of components (A)     and (B) being used in a concentration of 10 to 500 ppm by weight,     based on the total amount of the fuel oil.

This mixture of components (A) and (B) in fuel oils in the presence of customary amounts of cetane number improvers and additives with detergent action increases the cetane number (determined to the standard EN ISO 5165) generally by at least 1.0 unit, usually even by at least 1.5 units, compared to the fuel oil without cetane number improver and without additives with detergent action. The corresponding increase in the cetane number (determined to the standard EN ISO 5165) is generally at least 0.5 unit compared to the fuel oil containing the same amount of cetane number improver and the same amount of additives with detergent action. The mixture of components (A) and (B), together with the cetane number improver, brings about a synergistic increase in the cetane number.

Mixtures of said saturated or unsaturated monocarboxylic acids having 12 to 24 carbon atoms or the dimerization or trimerization products thereof (A) and said polycyclic carbon compounds (B), which are obtainable from distillation residues of natural oils which have been extracted from tree resins, are described in WO 2007/082825 for improvement of the storage stability of fuel additive concentrates which comprise at least one detergent and at least one cetane number improver.

Component (A) in the mixtures mentioned comprises preferably aliphatic saturated or unsaturated monocarboxylic acids having 14 to 20 carbon atoms, especially 16 to 18 carbon atoms. These monocarboxylic acids are generally linear. Useful for component (A) are especially naturally occurring fatty acids, in particular those having 14 to 20 carbon atoms, especially 16 to 18 carbon atoms. Typical representatives of such monocarboxylic acids or fatty acids are lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid and elaidic acid. Component (A) may consist only of one such monocarboxylic acid or fatty acid or preferably of a mixture of two or more such monocarboxylic acids or fatty acids. In the case of naturally occurring fatty acids, as obtained, for example, from rapeseed oil, soybean oil or tall oil, these are generally mixtures of several such monocarboxylic acids.

Component (B), which naturally originates from tree resins, especially conifer resins from pines or spruces, is formed from one or preferably more than one so-called resin acid. Resin acids are carboxyl-containing polycyclic hydrocarbon compounds. They include, as the most important representatives, abietic acid, dehydroabietic acid, dihydroabietic acid, tetrahydroabietic acid, neoabietic acid, palustric acid, pimaric acid, isopimaric acid and levopimaric acid. These resin acids may partly also be present in oxidized form as so-called oxy acids.

In a preferred embodiment, components (A) and (B) are used in the mixtures to be used in accordance with the invention in a weight ratio of 65 to 99.9:0.1 to 35, especially of 90 to 99.9:0.1 to 10, in particular of 97 to 99.9:0.1 to 3.

Particularly suitable mixtures of components (A) and (B) are those of tall oil fatty acid and dimerized tall oil fatty acid. Tall oil fatty acid is produced from tall oil, which is obtained by digestion of resin-rich wood types, especially of spruce or pine wood. Tall oil fatty acid is a mixture of fatty acids in which the C₁₈-unsaturated monocarboxylic acids, in particular oleic acid, linoleic acid and conjugated C₁₈ fatty acids, and also 5,9,12-octadecatrienoic acid, predominate, resin acids and optionally oxyacids (i.e. oxidized fatty acids and resin acids). Resin acids are so-called tall resin, in which abietic acid, dehydroabietic acid and palustric acid predominate, and smaller proportions of dihydroabietic acid, neoabietic acid, pimaric acid and isopimaric acid can be found as well as further resin acids. In the best tall oil fatty acid quality, the fatty acid content is at least 97% by weight and the tall resin content is up to 3% by weight.

The recovery of tall oil fatty acid and resin acids from resin trees by digestion, extraction and distillation processes is known to those skilled it the art and therefore need not be explained any further here.

In dimerized tall oil fatty acid, the fatty acid component (A) is present in dimerized form. Dimerizations and trimerizations of monocarboxylic acids or fatty acids can be performed by processes customary for this purpose and are known in principle to those skilled in the art.

The monocarboxylic acids or fatty acids and their dimerization or trimerization products of component (A) may be present as free carboxylic acids and/or as ammonium salts, for example as NH₄ salts or substituted ammonium salts such as mono-, di-, tri- or tetramethylammonium salts, and/or in the form of amides, esters or nitriles. Amide structures typical thereof have the —CO—NH₂, —CO—NH-alkyl or —CO—N(alkyl)₂ moieties, where “alkyl” here represents especially C₁- to C₄-alkyl radicals such as methyl or ethyl. Ester structures typically include C₁- to C₄-alkanol ester radicals such as methyl or ethyl ester radicals.

Additives with detergent action refer, in the context of the present invention to those compounds whose effect in an internal combustion engine, especially a diesel engine, consists predominantly or at least essentially of eliminating and/or preventing deposits. The detergents are preferably amphiphilic substances which have at least one hydrophobic hydrocarbyl radical having a number-average molecular weight (M_(n)) of 85 to 20 000, especially of 300 to 5000, and in particular of 500 to 2500, and at least one polar moiety.

In a preferred embodiment, the fuel oils comprise at least one additive with detergent action which is selected from

-   (i) compounds with moieties derived from succinic anhydride and     having hydroxyl and/or amino and/or amido and/or imido groups; -   (ii) nitrogen compounds quaternized in an acid-free manner,     obtainable by addition of a compound comprising at least one oxygen-     or nitrogen-containing group reactive with an anhydride and     additionally at least one quaternizable amino group onto a     polycarboxylic anhydride compound and subsequent quaternization; -   (iii) polytetrahydrobenzoxazines and bistetrahydrobenzoxazines.

Additives comprising moieties deriving from succinic anhydride and having hydroxyl and/or amino and/or amido and/or imido groups are preferably corresponding derivatives of polyisobutenylsuccinic anhydride, which are obtainable by reaction of conventional or high-reactivity polyisobutene with M_(n)=300 to 5000, in particular with M_(n)=500 to 2500, with maleic anhydride by a thermal route or via the chlorinated polyisobutene. Of particular interest in this context are derivatives with aliphatic polyamines such as ethylenediamine, diethylenetriamine, triethylenetetramine or tetraethylenepentamine. The moieties with hydroxyl and/or amino and/or amido and/or imido groups are for example carboxylic acid groups, acid amides, acid amides of di- or polyamines, which, as well as the amide function, also have free amine groups, succinic acid derivatives with an acid and an amide function, carboxymides with monoamines, carboxymides with di- or polyamines, which, as well as the imide function, also have free amine groups, and diimides, which are formed by the reaction of di- or polyamines with two succinic acid derivatives. Such fuel additives are described especially in U.S. Pat. No. 4,849,572.

Nitrogen compounds quaternized in an acid-free manner according to the above group (ii), which are obtainable by addition of a compound which comprises at least one oxygen- or nitrogen-containing group reactive with an anhydride and additionally at least one quaternizable amino group onto a polycarboxylic anhydride compound and subsequent quaternization, especially with an epoxide in the absence of free acid, are described in EP patent application 10 168 622.8. Suitable compounds having at least one oxygen- or nitrogen-containing group reactive with anhydride and additionally at least one quaternizable amino group are especially polyamines having at least one primary or secondary amino group and at least one tertiary amino group. Useful polycarboxylic anhydrides are especially dicarboxylic acids such as succinic acid, having a relatively long-chain hydrocarbyl substituent, preferably having a number-average molecular weight M_(n) for the hydrocarbyl substituent of 200 to 10 000, in particular of 350 to 5000. Such a quaternized nitrogen compound is, for example, the reaction product, obtained at 40° C., of polyisobutenylsuccinic anhydride, in which the polyisobutenyl radical typically has an M_(n) of 1000, with 3-(dimethylamino)propylamine, which constitutes a polyisobutenylsuccinic monoamide and which is subsequently quaternized with styrene oxide in the absence of free acid at 70° C.

Polytetrahydrobenzoxazines and bistetrahydrobenzoxazines according to the above group (iii) are described in EP patent application 10 194 307.4. Such polytetrahydrobenzoxazines and bistetrahydrobenzoxazines are obtainable by successively reacting, in a first reaction step, a C₁- to C₂₀-alkylenediamine having two primary amino functions, e.g. 1,2-ethylenediamine, with a C₁- to C₁₂-aldehyde, e.g. formaldehyde, and a C₁- to C₈-alkanol at a temperature of 20 to 80° C. with elimination and removal of water, where both the aldehyde and the alcohol can each be used in more than twice the molar amount, especially in each case in 4 times the molar amount, relative to the diamine, in a second reaction step reacting the condensation product thus obtained with a phenol which bears at least one long-chain substituent having 6 to 3000 carbon atoms, e.g. a tert-octyl, n-nonyl, n-dodecyl or polyisobutyl radical having an M_(n) of 1000, in a stoichiometric ratio relative to the originally used alkylenediamine of 1.2:1 to 3:1 at a temperature of 30 to 120° C. and optionally in a third reaction step heating the bistetrahydrobenzoxazine thus obtained to a temperature of 125 to 280° C. for at least 10 minutes.

The at least one additive with detergent action used for the present invention is more preferably a compound from group (i), which is a polyisobutenyl-substituted succinimide.

Cetane number improvers used are typically organic nitrates. Such organic nitrates are especially nitrate esters of unsubstituted or substituted aliphatic or cycloaliphatic alcohols, usually having up to about 10, in particular having 2 to 10 carbon atoms. The alkyl group in these nitrate esters may be linear or branched, and saturated or unsaturated. Typical examples of such nitrate esters are methyl nitrate, ethyl nitrate, n-propyl nitrate, isopropyl nitrate, allyl nitrate, n-butyl nitrate, isobutyl nitrate, sec-butyl nitrate, tert-butyl nitrate, n-amyl nitrate, isoamyl nitrate, 2-amyl nitrate, 3-amyl nitrate, tert-amyl nitrate, n-hexyl nitrate, n-heptyl nitrate, sec-heptyl nitrate, n-octyl nitrate, 2-ethylhexyl nitrate, sec-octyl nitrate, n-nonyl nitrate, n-decyl nitrate, cyclopentyl nitrate, cyclohexyl nitrate, methylcyclohexyl nitrate and isopropylcyclohexyl nitrate and also branched decyl nitrates of the formula R¹R²CH—CH₂—O—NO₂ in which R¹ is an n-propyl or isopropyl radical and R² is a linear or branched alkyl radical having 5 carbon atoms, as described in WO 2008/092809. Additionally suitable are, for example, nitrate esters of alkoxy-substituted aliphatic alcohols such as 2-ethoxyethyl nitrate, 2-(2-ethoxyethoxy)ethyl nitrate, 1-methoxypropyl nitrate or 4-ethoxybutyl nitrate. Additionally suitable are also diol nitrates such as 1,6-hexamethylene dinitrate. Among the cetane number improver classes mentioned, preference is given to primary amyl nitrates, primary hexyl nitrates, octyl nitrates and mixtures thereof.

In one preferred embodiment, 2-ethylhexyl nitrate is present in the fuel oils as the sole cetane number improver or in a mixture with other cetane number improvers.

Said mixtures of the monocarboxylic acids or the dimerization or trimerization products thereof (A) and the polycyclic hydrocarbon compounds (B) can in principle be used to increase the cetane numbers in any fuel oils which comprise cetane number improvers and additives with detergent action. However, they are especially suitable for use in middle distillate fuels, especially in diesel fuels. However, use in heating oil or kerosene is also possible. Diesel fuels or middle distillate fuels are typically mineral oil raffinates which generally have a boiling range from 100 to 400° C. These are usually distillates having a 95% point up to 360° C. or even higher. However, these may also be what is called “ultra low sulfur diesel” or “city diesel”, characterized by a 95% point of, for example, not more than 345° C. and a sulfur content of not more than 0.005% by weight, or by a 95% point of, for example, 285° C. and a sulfur content of not more than 0.001% by weight. In addition to the diesel fuels obtainable by refining, the main constituents of which are relatively long-chain paraffins, those obtainable by coal gasification or gas liquefaction [“gas to liquid” (GTL) fuels] are suitable. Also suitable are mixtures of the aforementioned diesel fuels with renewable fuels (biofuel oils) such as biodiesel or bioethanol. Of particular interest at present are diesel fuels with low sulfur content, i.e. with a sulfur content of less than 0.05% by weight, preferably of less than 0.02% by weight, particularly of less than 0.005% by weight and especially of less than 0.001% by weight of sulfur. Diesel fuels may also comprise water, for example in an amount of up to 20% by weight, for example in the form of diesel-water microemulsions or in the form of what is called “white diesel”.

In a preferred embodiment, said mixtures of the monocarboxylic acids or the dimerization or trimerization products thereof (A) and the polycyclic hydrocarbon compounds (B) are used together with cetane number improvers and additives with detergent action in fuel oils which consist

-   (a) to an extent of 0.1 to 100% by weight, preferably to an extent     of 0.1 to less than 100% by weight, especially to an extent of 10 to     95% by weight and in particular to an extent of 30 to 90% by weight,     of at least one biofuel oil based on fatty acid esters, and -   (b) to an extent of 0 to 99.9% by weight, preferably to an extent of     more than 0 to 99.9% by weight, especially to an extent of 5 to 90%     by weight, and in particular to an extent of 10 to 70% by weight, of     middle distillates of fossil origin and/or of vegetable and/or     animal origin, which are essentially hydrocarbon mixtures and are     free of fatty acid esters.

Said mixtures of components (A) and (B) can of course also be used together with cetane number improvers and additives with detergent action in fuel oils which consist to an extent of 100% by weight of at least one biofuel oil (a), based on fatty acid esters.

The fuel oil component (a) is usually also referred to as “biodiesel”. This preferably comprises essentially alkyl esters of fatty acids which derive from vegetable and/or animal oils and/or fats. Alkyl esters typically refer to lower alkyl esters, especially C₁- to C₄-alkyl esters, which are obtainable by transesterifying the glycerides which occur in vegetable and/or animal oils and/or fats, especially triglycerides, by means of lower alcohols, for example, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, tert-butanol or especially methanol (“FAME”).

Examples of vegetable oils which can be converted to corresponding alkyl esters and can thus serve as the basis of biodiesel are castor oil, olive oil, peanut oil, palm kernel oil, coconut oil, mustard oil, cottonseed oil, and especially sunflower oil, palm oil, soybean oil and rapeseed oil. Further examples include oils which can be obtained from wheat, jute, sesame and shea tree nut; it is additionally also possible to use arachis oil, jatropha oil and linseed oil. The extraction of these oils and the conversion thereof to the alkyl esters are known from the prior art or can be inferred therefrom.

It is also possible to convert already used vegetable oils, for example used deep fat fryer oil, optionally after appropriate cleaning, to alkyl esters, and thus for them to serve as the basis of biodiesel.

Vegetable fats can in principle likewise be used as a source for biodiesel, but play a minor role.

Examples of animal oils and fats which can be converted to corresponding alkyl esters and can thus serve as the basis of biodiesel are fish oil, bovine tallow, porcine tallow and similar fats and oils obtained as wastes in the slaughter or utilization of farm animals or wild animals.

The parent saturated or unsaturated fatty acids of said vegetable and/or animal oils and/or fats, which usually have 12 to 22 carbon atoms and may bear an additional functional group such as hydroxyl groups, and which occur in the alkyl esters, are especially lauric acid, myristic acid, palmitic acid, stearic acid, oleic acid, linoleic acid, linolenic acid, elaidic acid, erucic acid and/or ricinoleic acid.

Typical lower alkyl esters based on vegetable and/or animal oils and/or fats, which find use as biodiesel or biodiesel components, are, for example, sunflower methyl ester, palm oil methyl ester (“PME”), soybean oil methyl ester (“SME”) and especially rapeseed oil methyl ester (“RME”).

However, it is also possible to use the monoglycerides, diglycerides and especially triglycerides themselves, for example castor oil, or mixtures of such glycerides, as biodiesel or components for biodiesel.

In the context of the present invention, the fuel oil component (b) shall be understood to mean the abovementioned middle distillate fuels, especially diesel fuels, especially those which boil in the range from 120 to 450° C.

In a further preferred embodiment, said mixtures of the monocarboxylic acids or the dimerization or trimerization products thereof (A) and the polycyclic hydrocarbon compounds (B) are used together with cetane number improvers and additives with detergent action in fuel oils which have at least one of the following properties:

-   (α) a sulfur content of less than 50 mg/kg (corresponding to 0.005%     by weight), especially less than 10 mg/kg (corresponding to 0.001%     by weight); -   (β) a maximum content of 8% by weight of polycyclic aromatic     hydrocarbons; -   (γ) a 95% distillation point (vol/vol) at not more than 360° C.

Polycyclic aromatic hydrocarbons in (13) shall be understood to mean polyaromatic hydrocarbons according to standard EN 12916. They are determined according to this standard.

The fuel oils comprise said mixtures of the monocarboxylic acids or the dimerization or trimerization products thereof (A) and the polycyclic hydrocarbon compounds (B) in the context of the present invention generally in an amount of 1 to 1000 ppm by weight, preferably of 5 to 500 ppm by weight, especially of 10 to 300 ppm by weight, in particular of 25 to 150 ppm by weight, for example of 40 to 100 ppm by weight.

The cetane number improver or a mixture of a plurality of cetane number improvers is present in the fuel oils normally in an amount of 10 to 10 000 ppm by weight, especially 20 to 5000 ppm by weight, even more preferably of 50 to 2500 ppm by weight and especially of 100 to 1000 ppm by weight, for example of 150 to 500 ppm by weight.

The additive with detergent action or a mixture of a plurality of such additives with detergent action is present in the fuel oils, typically in an amount of 10 to 2000 ppm by weight, especially 20 to 1000 ppm by weight, even more preferably of 50 to 500 ppm by weight and especially of 30 to 250 ppm by weight, for example of 50 to 150 ppm by weight.

Said fuel oils such as diesel fuels or middle distillate fuels, or such as said mixtures of biofuel oils and middle distillates of fossil, vegetable or animal origin, may comprise, in addition to the mixtures of the monocarboxylic acids or the dimerization or trimerization products thereof (A) and the polycyclic hydrocarbon compounds (B), the additives with detergent action and the cetane number improvers, as coadditives, further customary additive components, especially cold flow improvers, corrosion inhibitors, demulsifiers, dehazers, antifoams, antioxidants and stabilizers, metal deactivators, antistats, lubricity improvers, dyes (markers) and/or diluents and solvents.

Cold flow improvers suitable as further coadditives are, for example, copolymers of ethylene with at least one further unsaturated monomer, in particular ethylene-vinyl acetate copolymers.

Corrosion inhibitors suitable as further coadditives are, for example, succinic esters, in particular with polyols, fatty acid derivatives, for example oleic esters, oligomerized fatty acids and substituted ethanolamines.

Demulsifiers suitable as further coadditives are, for example, the alkali metal and alkaline earth metal salts of alkyl-substituted phenol- and naphthalenesulfonates and the alkali metal and alkaline earth metal salts of fatty acid, and also alcohol alkoxylates, e.g. alcohol ethoxylates, phenol alkoxylates, e.g. tert-butylphenol ethoxylates or tert-pentylphenol ethoxylates, fatty acid, alkylphenols, condensation products of ethylene oxide and propylene oxide, e.g. ethylene oxide-propylene oxide block copolymers, polyethyleneimines and polysiloxanes.

Dehazers suitable as further coadditives are, for example, alkoxylated phenol-formaldehyde condensates.

Antifoams suitable as further coadditives are, for example, polyether-modified polysiloxanes.

Antioxidants suitable as further coadditives are, for example, substituted phenols, e.g. 2,6-di-tert-butylphenol and 2,6-di-tert-butyl-3-methylphenol, and also phenylenediamines, e.g. N,N′-di-sec-butyl-p-phenylenediamine.

Metal deactivators suitable as further coadditives are, for example, salicylic acid derivatives, e.g. N,N′-disalicylidene-1,2-propanediamine.

A lubricity improver suitable as a further coadditive is, for example, glyceryl monooleate.

Suitable solvents, especially for diesel performance packages, are, for example, nonpolar organic solvents, especially aromatic and aliphatic hydrocarbons, for example toluene, xylenes, “white spirit” and the technical solvent mixtures of the designations Shellsol® (manufacturer: Royal Dutch/Shell Group), Exxol® (manufacturer: ExxonMobil) and Solvent Naphtha. Also useful here, especially in a blend with the nonpolar organic solvents mentioned, are polar organic solvents, in particular alcohols such as 2-ethylhexanol, decanol and isotridecanol.

When the coadditives and/or solvents mentioned are used additionally, they are used in the amounts customary therefor.

The examples which follow are intended to illustrate the present invention without restricting it.

EXAMPLES

In a diesel fuel which is typical for the European market, conforms to standard EN 590 and comprised a proportion of 7% by weight of biodiesel (FAME), the cetane numbers were determined to EN ISO 5165 with the following additions:

Sample Cetane number to No. Dosage [ppm by weight] EN ISO 5165 1 none (base fuel) 51.9 2 65 PIBSI * 52.1 0 2-ethylhexyl nitrate 60 tall oil fatty acid 110 Solvent Naphtha heavy Tot. 5 customary antifoam + customary dehazer 3 65 PIBSI * 52.9 360 2-ethylhexyl nitrate 0 tall oil fatty acid 110 Solvent Naphtha heavy Tot. 5 customary antifoam + customary dehazer 4 65 PIBSI * 53.6 360 2-ethylhexyl nitrate 60 tall oil fatty acid 110 Solvent Naphtha heavy Tot. 5 customary antifoam + customary dehazer * commercial polyisobutenyl-substituted succinimide (Kerocom ® PIBSI from BASF SE)

As evident from the above results, the addition of tall oil fatty acid to a fuel which comprises an additive with detergent action but no cetane number improver does not lead to any significant change in the cetane number (sample No. 2). The addition of cetane number improver to a fuel which comprises an additive with detergent action but no tall oil fatty acid leads to an increase in the cetane number of 1.0 compared to the unadditized base fuel (sample No. 3). When, in contrast, both tall oil fatty acid and cetane number improver are added to the fuel which comprises an additive with detergent action, there is a surprisingly large increase in the cetane number by 1.7 units compared to the unadditized base fuel to 53.6 (sample No. 4).

This demonstrates the synergistic action of the mixture of components (A) and (B), represented by tall oil fatty acid, and cetane number improvers from the increase in the cetane number. 

1. The use of mixtures of A) aliphatic saturated or unsaturated monocarboxylic acids having 12 to 24 carbon atoms or the dimerization or trimerization products thereof, which may be present in the form of free carboxylic acids and/or in the form of ammonium salts, amides, esters and/or nitriles, and B) polycyclic hydrocarbon compounds which are obtainable from distillation residues of natural oils, which have been extracted from tree resins, for increasing the cetane number of fuel oils which comprise at least one additive with detergent action and at least one cetane number improver, the mixtures of components (A) and (B) being used in a concentration of 10 to 500 ppm by weight, based on the total amount of the fuel oil.
 2. The use according to claim 1, in which the fuel oils comprise at least one additive with detergent action which is selected from (i) compounds with moieties derived from succinic anhydride and having hydroxyl and/or amino and/or amido and/or imido groups; (ii) nitrogen compounds quaternized in an acid-free manner, obtainable by addition of a compound comprising at least one oxygen- or nitrogen-containing group reactive with an anhydride and additionally at least one quaternizable amino group onto a polycarboxylic anhydride compound and subsequent quaternization; (iii) polytetrahydrobenzoxazines and bistetrahydrobenzoxazines.
 3. The use according to claim 2, in which the fuel oils comprise at least one polyisobutenyl-substituted succinimide as an additive with detergent action.
 4. The use according to claims 1 to 3, in which the fuel oils comprise 2-ethylhexyl nitrate as a cetane number improver.
 5. The use according to claims 1 to 4, in which the carboxylic acids (A) and the polycyclic hydrocarbon compounds (B) are present in the mixtures in a weight ratio relative to one another of 65 to 99.9:0.1 to 35, especially of 90 to 99.9:0.1 to
 10. 6. The use according to claims 1 to 5, in which the mixtures of carboxylic acids (A) and polycyclic hydrocarbon compounds (B) used are tall oil fatty acid or dimerized tall oil fatty acid.
 7. The use according to claims 1 to 6 for use in fuel oils, which consist (a) to an extent of 0.1 to 100% by weight of at least one biofuel oil based on fatty acid esters, and (b) to an extent of 0 to 99.9% by weight of middle distillates of fossil origin and/or of vegetable and/or animal origin, which are essentially hydrocarbon mixtures and are free of fatty acid esters.
 8. The use according to claims 1 to 6 for use in fuel oils which have at least one of the following properties: (α) a sulfur content of less than 50 mg/kg; (β) a maximum content of 8% by weight of polycyclic aromatic hydrocarbons; (γ) a 95% distillation point (vol/vol) at not more than 360° C. 